A double barrel occurs when 2 men are docking and one of the men ejaculates into the others dickhole. The man with the "loaded barrel" then promptly ejacutes into the closest fertile female In hopes of simutaneously impregnating her with both of the males seeds.

My homosexual friend who wants to be a father and he approached me with a double barrel offer.

The woman who recently gave birth to twins with two father's, must have seen some double barrel action.